The use of heat for the cauterization of bleeding wounds dates back to ancient times. Perhaps the simplest and most basic thermal cauterization technique involves the application of a hot iron to a bleeding wound. While this technique is somewhat effective in cauterizing large, external wounds, the technique is not applicable to internal wounds. Nor is the technique sufficiently precise or delimited to provide adequate cauterization without excessive tissue damage.
In the present century, the use of radio frequency electric current traveling through a portion of the body has been widely used to stop bleeding. The essential ingredient in radio frequency cauterization is the dissipation of electrical energy in resistive tissue. This dissipated electrical energy is converted into heat, which produces a rise in temperature of the tissue and blood. The plasma proteins in blood are denatured in a temperature range of from 50.degree. to 100.degree. C., producing a sticky or congealed mass of protein. This process is familiar in the cooking of egg white. Other processes may take place when tissue is heated. For example, vessels may contract or shrink, thereby further reducing the flow of blood.
Several radio frequency current generators are now commercially available and are widely used by surgeons for both cutting and coagulating tissue. Since the electrical current flow follows the path of least resistance, the resulting thermal damage, or necrosis, may at times be unpredictable, too deep and uncontrolled. The rationale for using radio frequency current for bleeding control is that the frequency is above that which would cause neuromuscular stimulation and yet permit sufficient power dissipation to produce a rapid rise in temperature. Thus, used properly, electrical shock does not occur and coagulation is accomplished.
There is currently much interest in the control of bleeding using the modern fiberoptic endoscope, which permits visualization and therapy in hollow organs of the body through a slender tube. Hollow channels with a few millimeters of inside diameter permit the insertion of instruments for the administration of therapy such as the coagulation of bleeding. Some investigators have reported good success using radio frequency coagulation through the endoscope in a clinical setting. But this technique has not been widely used in practice because of its inherent risks. Several groups have directed a laser beam through an endoscope using a special optical wave-guide with good success in both animals and humans. However, the high cost of laser coagulators and the as-yet unproven benefit in a controlled clinical trial are slowing the widespread adoption of this technique. Other problems associated with laser coagulators arise from the difficulty in precisely directing the laser beam to a moving target, the existence of optical hazards and the need for a gas injection system to wash away overlying blood. Furthermore, simple laser coagulators do not simultaneously apply heat and pressure to the wound; and the combination of heat and pressure is considered to be more effective than heat alone.
More recently, a miniaturized thermal probe has been developed which is endoscopically deliverable. This probe, which is described in an article by Protell, et al., "The Heater Probe: A New Endoscopic Method for Stopping Massive Gastro-Intestinal Bleeding" Gastroenterology, 74: 257-62 (1978), includes a heating coil mounted in a small cylindrical body with a thermocouple. The output of the thermocouple is compared to a temperature reference level, and the difference is used to control the power to the probe to achieve a preset probe temperature. In use, the probe is heated to the preset value and applied to the wound for a number of periods, each of approximately one second in duration. Alternatively, the cold probe is applied directly to the bleeding site, turned on and held there for a predetermined period after reaching a target temperature. The principal problem associated with the latter technique is the inability of the probe to reach coagulating temperature with sufficient speed and to then cool itself with sufficient speed to prevent excessive penetration of the heat by diffusion. Effective coagulation requires that the bleeding site be adequately heated. However, avoidance of thermal necrosis requires that the heat not penetrate too deeply. The only technique providing adequate heating of the bleeding site without producing excessive heat penetration is heating the bleeding site at a high temperature for an extremely short period of time. Presently existing thermal probes are not able to meet these requirements. The problem does not stem from an inability to heat the probe with sufficient speed as much as it does from an inability to cool the probe with sufficient speed. Any probe can be heated rapidly by merely utilizing a sufficiently larger heater. However, the probe can be cooled only by the tissue with which it is in contact. Conventional probes have been incapable of being cooled by the surrounding tissue with sufficient speed due to their relatively high thermal mass.
Attempts have been made to design thermal cautery probes which are heated by passing a current through the body of the probe itself instead of through a separate heating element. An example of such probes is disclosed in U.S. Pat. No. 3,886,944, issued to Jamshidi. The disadvantages of such probes are twofold: first, the unavailability of a satisfactory probe material and, second, the nonuniformity of the probe temperature.
The choice of a probe material presents a problem because the resistance of the material must be high enough to dissipate sufficient power and the strength of the material must be high enough to withstand forces applied to the probe by the tissue and other objects. The Jamshidi probe utilizes a Nichrome alloy or stainless steel as the probe material. Either material has a relatively low resistivity, thereby making it difficult for the probe to dissipate sufficient power without applying a great deal of current to the probe. While probes requiring high current are acceptable under some circumstances, they are uncceptable where the probe is to be endoscopically deliverable since the high currents require wires which are larger than the endoscope channels. In fact, a probe having a resistance less than about 0.5 ohm will generally require more current than endoscopically deliverable power leads are capable of carrying.
A probe fabricated of a low-resistivity material can dissipate adequate power from relatively low current only by making the material extremely thin so that the resistance of the probe is high. Yet a probe having an extremely thin shell does not have sufficient strength to withstand clinical use.
A probe having a relatively thick shell of a higher resistivity or semiconductive material would be capable of dissipating adequate power at acceptably low currents. However, a material having these properties and which is inexpensive, easily worked, and sufficiently sturdy does not appear to be available.
The second disadvantage mentioned above--the nonuniformity of probe temperature--is illustrated in the Jamshidi patent. In the Jamshidi probe, current flows outwardly from the center of the probe tip and then along the sides of the probe. The current density--and hence the power dissipation--varies from a maximum at the center of the probe to a minimum at the sides of the probe. As a result, the temperature of the probe decreases from a maximum at the center of the probe.